Marine seismic exploration investigates and maps the structure and character of subsurface geological formations underlying a body of water. Marine seismic data is typically gathered by towing seismic sources (e.g., air guns) and seismic receivers (e.g., hydrophones) through a body of water behind one or more marine vessels. As the seismic sources and receivers are towed through the water, the seismic sources generate acoustic energy that travel through the water and into the earth, where they are reflected and refracted by interfaces between subsurface geological formations. The seismic receivers detect the resulting reflected and refracted energy, thus acquiring seismic data that provides seismic information about the geological foundations underlying the body of water.
Typically, large arrays of seismic receivers, often numbering in the thousands, are used to gather marine seismic data. The seismic receivers are generally attached to and spaced apart along streamer cables that are towed behind a marine vessel.
By way of illustration of such a system, FIG. 1 shows a simplified depiction of a conventional marine seismic data acquisition system employing a marine vessel 10 to tow seismic sources 12 and a system 14 of steerable seismic streamers 16 through a body of water 18.
Each of seismic streamers 16 includes a streamer cable 20, a series of seismic receivers 22 and a series of steering devices 24 coupled to cable 20. Relative positions of the marine seismic receivers during seismic data acquisition can affect the quality and utility of the resulting seismic data. However, unpredictable environmental forces such as currents, winds, and sea states present in many marine environments can cause the relative positions of marine seismic receivers to vary greatly as they are towed through the water. Therefore, it is common for steering devices (commonly know as “birds”) to be attached to the streamer cables so that the relative positions (both lateral and vertical) of the seismic receivers can be controlled as they are towed through the water. As depicted in FIG. 1, during conventional marine seismic acquisition, steering devices 24 are used to maintain substantially constant lateral spacing between seismic streamers 16.
As a further illustration of typical marine seismic systems, FIG. 2 illustrates a side view of marine vessel 10 towing one or more streamers 12 having seismic sources 12 (□) and/or seismic receivers 22 (◯) through body of water 18 to acquire seismic data for a subterranean geological formation region of interest 26 of geological formation 25.
As marine vessel 10 tows seismic sources 12 and receivers 22 through body of water 18, seismic sources 12 are simultaneously excited, which generate acoustic wave energy that propagates down through water 18 and into geological formation 25. The acoustic wave energy is then reflected and refracted by interfaces between strata of geological formation 25. The resulting reflected/refracted seismic energy then passes upwardly through water 18 and is detected by seismic receivers 22. Additional passes are then conducted to survey additional points of interest. The seismic data detected by seismic receivers 22 then provides seismic information representative of subterranean geological formation of interest 26.
A common problem encountered with conventional marine seismic surveys is “gaps” in the acquired seismic data. These data gaps can occur when the spacing between adjacent acquisition passes is too large to provide sufficient resolution for proper data processing. Gaps in seismic data can be caused by a number of factors including, for example, skewing of the seismic streamers relative to the direction of travel of the towing vessel during data acquisition. Even when steerable streamers are employed, gaps in seismic data are common, particularly when strong crosscurrents are present. When strong crosscurrents are present during seismic data acquisition, it is not practical to maintain all the streamers in desired orientation, because fighting strong crosscurrents with steering devices may produce noise that dramatically reduces the quality of the gathered seismic data.
When gaps in marine seismic data are discovered, if the data gaps cannot be filled by post-acquisition interpolation methods, the areas corresponding to the data gaps must be resurveyed, a process commonly known as “shooting in-fill” or “in-filling.” Unfortunately, the existence of gaps in marine seismic data may not be discovered until the initial marine seismic survey has been completed and the resulting seismic data is being processed. Obviously, in-filling is highly undesirable because of the significant expense and time involved in resurveying in-fill areas that may be located hundreds of kilometers from one another or even retransiting the same vessel pass again to make up coverage.
Traditionally, marine seismic surveys using the systems depicted in FIGS. 1 and 2 above are conducted using a series of straight line sail paths across a region on interest. That is, under conventional methods, a marine vessel and its corresponding streamers sail back and forth across a geological region of interest, incrementally moving each subsequent pass or sweep over slightly until all of the combined paths have covered the survey region of interest. In this way, traditional seismic surveys follow a survey path similar to the path followed by one mowing a rectangular section of lawn with a lawn mower, namely, a back and forth straight line path that is moved over incrementally each pass until the entire section of lawn is covered.
Referring again to FIG. 1, traditional marine seismic survey systems employ a set of streamers where the lateral distance (df) of the forward-most seismic receivers is equivalent to the lateral distance (dr)) of the rearwardly-most seismic receivers. Thus, in surveying a region of interest, a marine vessel 10 will typically employ a back and forth path across a geological region of interest, moving each pass or sweep over by roughly a distance of ½ df to a distance of about 1 df until the entire region of interest is surveyed. As will be seen below, this method of surveying suffers from a poor randomization and distribution of source point locations and receivers throughout the survey area. For example, for a ten streamer setup with dual sources towed by the streamer vessel, df might be about 900 m but each sail line would move over about 500 m. Accordingly, this poor randomization and distribution results in a decreased effectiveness of post-acquisition interpolation methods for filling in seismic data gaps in the acquired data.
Consequently, this method of surveying with a series of straight paths across a region is a highly inefficient way of gathering off-set and azimuth distributions. Using conventional methods to acquire wide azimuth distributions requires multiple passes down the same line with multiple boats, usually a single streamer vessel and multiple source vessels or two streamer vessels and multiple source vessels. Even using multiple passes and multiple vessels, the azimuth distribution acquired is still limited in certain directions. In this way, conventional methods of seismic surveys fail to provide full offset and azimuth data and further fail to optimize the randomization of the offset and azimuth data available. Accordingly, conventional methods of surveying a region are unnecessarily more costly in terms of both time and direct survey costs. Indeed, the cost of acquiring wide-azimuth data essentially increases by the cost of the number of passes required down each sail line to obtain the azimuth range required. For conventional marine surveys that are not wide azimuth, costs can be increased by as much as 50% by infill needs.
Accordingly, there is a need in the art for improved seismic survey methods and systems that address one or more disadvantages of the prior art.